1. Field of the Invention
This invention relates to directed infrared countermeasure (DIRCM) systems for use with commercial aircraft to effectively counter MANPADS and more advanced threats, and more particularly to a transmit telescope for directing the laser on target
2. Description of the Related Art
The proliferation of shoulder-launched missiles known as MANPADS for “Man-Portable Air-Defense System” and their availability to terrorists present a real threat to military aircraft and particularly commercial aircraft. Estimates of the number of attacks on commercial aircraft vary, running as high as 43 hits on civilian aircraft—with 30 of these resulting in aircraft kills and the loss of nearly 1,000 lives—since the 1970s. More than half a million MANPADS have been delivered worldwide, and many of these are still operational. These missiles currently use infrared (IR) seekers to track and lock-on to the aircraft. The missiles typically have a range of 5-8 km and can reach an altitude of approximately 12,000 ft. Historically, countermeasures range from active IR jamming to flares and chaff.
As illustrated in FIGS. 1 and 2, a terrorist 10 holds a MANPADS 12 on his or her shoulder, points it at the aircraft 14 and launches the missile 16. A typical missile 16 will typically progress through the eject, boost, sustain and possibly post-burn stages before impacting the aircraft. The missile's IR seeker 18 tracks IR energy emitted by the aircraft 14. The seeker processes the infrared scene containing the target and generates target tracking information 22 that guides the missile 16 enabling the seeker to track hot targets like aircraft 14. The aircraft's DIRCM system 24, suitably mounted in a “blister pack” near the rear of the aircraft, must detect, verify, track and then emit a modulated laser beam 26 or eject flares that produce a false signature 28 to jam the missile's IR seeker. The purpose of either approach is to generate a false target with a “miss distance” from the aircraft. The DIRCM system will typically try to detect the missile at ejection based on the eject motor's impulse signature, verify the threat and track the heat plume 30. The DIRCM system is particularly stressed when the shot is taken from close range such as might be the case on take-off or landing or when multiple simultaneous shots are taken at the aircraft. This later case being taught to terrorists undergoing formal training.
U.S. Pat. No. 7,378,626 discloses an agile, high-power, reliable DIRCM system that is easily extended to address sophisticated UV or UV-visible capable multi-band threats. The DIRCM system includes a missile warner having missile warning receivers (MWRs), one or two-color suitably in the mid-IR range, that detect likely missile launch and pass the threat coordinates to a pointer-tracker having a Roll/Nod gimbal on which the IR laser transmitter is mounted. The pointer-tracker stews the gimbal to initiate tracking based on the threat coordinates and then uses its detector to continue to track and verify the threat. If the threat is verified, the pointer-tracker engages the laser to fire and jam the missile's IR seeker. By slewing the gimbal based on unverified threat coordinates to initiate tracking the system is highly agile and can respond to short and near simultaneous MANPADS shots. The laser transmit telescope(s) is mounted off-axis from the Roll/Nod gimbal axes (i.e. a lateral offset from optical axis of the receive telescope) to avoid backscatter from the laser into the pointer-tracker's detector. Conformal optics (multi-element on-axis refractive optics) is used to minimize the distortion through the spherical dome caused by the off-axis mounting. U.S. Pat. No. 7,304,296 discloses a continuous optical fiber assembly to couple the off-gimbal laser to the on-gimbal transmit telescope.